A cerium oxide powder is a high functional ceramic powder widely used as a raw material for polishing materials, catalysts, fluorescent bodies, and the like, and has recently gained interest as an inorganic polishing material, which is a key raw material of a polishing solution for next generation CMP (Chemical Mechanical Polishing) used in an STI (Shallow Trench Isolation) process or the like, in the field of semiconductor elements.
The CMP solution used in an STI CMP process is very important for the polishing speed of a silicon oxide (SiO2) film, the selective polishing characteristics between a silicon oxide film and a silicon nitride (Si3N4) film, and the scratch prevention characteristics of a polished surface. In order to realize the such removal performance, mechanical/chemical properties of the cerium oxide powder used as a polishing material should be controlled. Particularly, when a cerium oxide polishing materials are applied to CMP slurry, the study of surface properties and hardness for the polishing material is actively developed as the importance of chemical reactions on a SiO2 film is emphasized.
Typically, the hardness of cerium oxide is lower than that of aluminum oxide (Al2O3) or silicon oxide (SiO2), which are all used as ceramic polishing materials. Nevertheless, the reason that cerium oxide is mainly used as a polishing material for a silicon oxide film or a silica-glass surface rather than the aforementioned materials is because the polishing speed is faster. This appears to be because a chemical polishing effect, as well as a mechanical polishing effect, acts when a silicon oxide film is polished by cerium oxide.
According to a research report about the chemical reactions of cerium oxide (Journal of Non-Crystalline Solid, 283 (2001) 129-136), when cerium oxide is used as a polishing material, unlike mechanical polishing for removing only a hydrated layer formed on a surface, it was shown that a silicon oxide film is polished by the chemical binding of Si—O—Ce due to the high reactivity of cerium oxide with silicon oxide. The cerium oxide removes silicon oxide lumps as if by plucking off from a silicon oxide film surface. Accordingly, in order to obtain a faster polishing speed, the chemical reactivity level, specific surface area, appropriate strength of the polishing particles and the like, should be controlled above anything else on a particle surface.
On the other hand, for a method of enhancing the chemical reactivity of ceramic particles, a method of increasing a grain boundary having high chemical activity is most representative. Ceramic particles have a characteristic of being more chemically stable by having a perfect crystalline structure as they enter into the particles. On the contrary, the grain boundary or surface has an imperfect crystalline structure because the crystalline structure cannot be fundamentally perfect, and also is in a high state of chemical reactivity to accept other elements or ions for supplementing such an imperfect crystalline structure. Therefore, when the cerium oxide powder has a higher specific surface area per gram, it will have a higher chemical reactivity and Si—O—Ce binding is generated over a broader area, thereby improving polishing speed.